your best bet is to either
1. wait and possibly never find one
2. buy one full pop at the dealer and then sell it a couple hundred bucks cheaper than what you bought it when you get rid of the car
I've been looking for an M exhaust or that market myself... and reality is that the second theres one available... its sold within a week
I'm most probably buying the M exhaust for the 340 next spring... yes it'll cost me 2k but I'll sell it for 1500-1600$ when I give the car back at the end of the lease
fortunately the M stuff holds value pretty good